ANATOMY AND PHYSIOLOGY 



FIG. 171. Caudal 

 gills of an agrionid 

 nymph, enlarged. 



surface; their integument is thin and the trachea? spread closely beneath 

 it. These adaptations are often supplemented by waving movements of 

 the gills, as in May fly nymphs, and by frequent movements of the insect 

 from one place to another. 



Especially noteworthy are the rectal tracked gills of odonate nymphs. 

 In these insects the lining of the rectum forms nu- 

 merous papillae or lamellae, which contain a profusion 

 of delicate tracheal branches; these are bathed by 

 water drawn into the rectum and then expelled, at 

 rather irregular intervals. A similar rectal respira- 

 tion occurs also in ephemerid nymphs and mosquito 

 larvae. 



A few forms, chiefly Perlidae, are exceptional in 



retaining tracheal gills in the adult stage; in some imagines they are merely 

 vestiges of the nymphal gills, but in others, such as Pteronarcys (Fig. 18), 

 which habitually dips into the water and rests in moist situations, the 



gills probably supplement the spiracles. Further 

 details on the respiration of aquatic insects are 

 given in Chapter IV. 



Spiracles. The paired external openings of the 

 tracheae occur on the sides of the thorax and ab- 

 domen', there being never more than one pair to a 

 segment. Though the thysanuran Japyx has n 

 pairs, no winged insect has more than 10; although 

 there are in all 1 2 segments which may bear spira- 

 cles the three thoracic and the first nine abdominal 

 segments. (Additional details are given on page p. 52.) 

 The spiracles, or stigmata, are usually provided 

 with bristles, hairs or other processes to exclude 

 dust ; or the hairs of the body may serve the same 

 purpose, as in Lepidoptera and Diptera; in many 

 beetles the spiracles are protected by the elytra; in 

 other beetles, however, and in many Hemiptera and 

 Diptera the spiracles are unprotected externally. 

 Larvae that live in water or mud may have spiracles 

 at the end of a long tube, which can be thrust up 

 into the pure air; this is true of the dipterous larvae of Erislalis, Bitta- 

 comorpha (Fig. 173) and Culex (Fig. 230). 



Closure of Spiracles. As a rule, a spiracle is opened and closed 

 periodically by means of a valve, operated by a special ocdusor muscle. 



FIG. 172. Cater- 

 pillar of Par a pony x 

 obscuralis, to show tra- 

 cheal gills. Length, 15 

 mm. After HART. 



